/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/MeshTangents.h
If tangents are available in the overlays, returns them, otherwise falls back to unit X/Y axes.
void GetTangentVectors ( int32 TriangleID, int32 TriVertexIndex, FVector3f & TangentOut, FVector3f & BitangentOut ) const